#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
typedef long long LL;
const int N = 2e5 + 10;
int n;
double d, h;
int a[N];
int main()
{
	int t;
	cin >> t;
	while (t--) {
		cin >> n >> d >> h;
		memset(a, 0, sizeof(a));
		for (int i = 1; i <= n; i++)cin >> a[i];
		double ans = (d * h * n);
		double sum = 0;
		sort(a + 1, a + n + 1);	
		for (int i = 1; i <= n - 1; i++) {
			if (a[i] + h > a[i + 1]) {
				double x = (a[i] + h) - a[i + 1];
				sum += (x * x * d) * 1.0 / h;
			}
		}
		ans -= sum;
		printf("%.7lf\n", ans / 2.0);
	}
	return 0;
}